1. Field of the Invention
The present invention relates to a data communication system. More particularly, it relates to a two-wire full duplex frequency division multiplex (FDM) modulator and demodulator (MODEM) system having echo cancellation means and mainly used for a relatively low speed data transmission system.
2. Description of the Related Art
For two-wire full duplex MODEM systems using telephone lines and having a relatively low data transmission rate, the CCITT recommends a FDM MODEM system according to, for example, a standard V.22 bis: 2400 bits per second duplex modem standardized for use in general switched telephone networks and leased circuits, and an echo cancellation MODEM system according to, for example, a standard V.26 ter.
In the FDM MODEM system, a frequency band of 0.3 to 3.4 kHz on the telephone line is divided into two. One divided frequency band is used as a transmission frequency band, and the other divided frequency band is used as a reception frequency band. Accordingly, the FDM MODEM system has a merit of a simple and accurate extraction of a received signal at a reception MODEM. On the other hand, since a precise frequency discrimination is needed, complex and costly filters must be provided in the MODEM.
The echo cancellation can be achieved by a digital signal processing technique, and thus the costly filters become unnecessary. Nevertheless, difficult digital processing is required and a high precision extraction of the received signal can not be realistically achieved.
As described above, both systems suffer from antinomic problems.
In addition, the public telephone transmission networks connect a variety of modems. In order to facilitate the connection between and through a variety of modems, the establishment of standard hardware for a modem system is strongly required.